1. Field of the Invention
The present invention relates to a photoelectric smoke detector used to a disaster monitoring system for notifying the occurrence of a fire by collecting smoke density data, and more specifically, to a photoelectric smoke detector for determining the abnormality of a light emitting element, a light receiving element, a test light emitting element and the like by the comparison of the level difference between a zero point detection signal when the light emitting element is emitted and a zero point detection signal when the light emitting element is turned-off with a preset level difference, and the like.
2. Description of the Related Art
Nowadays, there have been widely used smoke detectors for detecting the occurrence of smoke in a monitored area and taking precautions against the possible spread of a fire. A photoelectric smoke detector is proposed as one of such smoke detectors.
A light emitting element and a light receiving element are disposed in the photoelectric smoke detector so that the optical axes thereof intersect with each other. An amount of the light received by the light receiving element when no smoke is generated is set to a zero point level. However, even if no smoke is generated, since the light generated by the light emitting element partly enters the light receiving element (which is referred to as noise light), the zero point level is not be perfect zero but has a positive value.
When smoke is generated, since the light scattered by the particles of the smoke is detected by the light receiving element, the amount of the light received by the light receiving element is increased from the zero point level. The photoelectric smoke detector detects the generation of smoke by detecting an increase of the amount of the light received by the light receiving element.
Conventionally, this type of the photoelectric smoke detector monitors the abnormality of light emitting elements, test light emitting elements and light receiving elements disposed therein as to the short circuit, release, deterioration and pollution thereof as well as carries out a test for confirming whether a peripheral circuit operates normally or not in order to ensure the detecting operation of the smoke detector when a fire happens.
Although the confirmation test includes a manual test effected by the use of a simple tester and an automatic test, when a lot of the photoelectric smoke detectors are deployed, the automatic test often being carried out because it is difficult to carry out the time-consuming manual test.
When a lot of the photoelectric smoke detectors are disposed in a disaster monitoring system, the automatic test is carried out using a control panel for receiving signals detected by the many photoelectric smoke detectors. More specifically, the control panel effects a polling control to send test commands to the respective photoelectric smoke detectors so that light is emitted by the light emitting elements. Then, whether the light emitted by the light emitting elements are properly received by the light receiving elements or not is confirmed to thereby test whether the light emitting elements, light receiving elements and a peripheral circuit disposed in the photoelectric smoke detector function normally or not. The operation of the light emitting elements, light receiving elements and the peripheral circuit disposed in the photoelectric smoke detectors is tested as described above.
In the above automatic test of the photoelectric smoke detector light from, the light emitting element is first emitted to obtain the aforesaid zero point level in the existence of noise light. Actually, the zero point level includes noise signals generated by the light receiving elements and amplifying circuit of the received lights in the respective photoelectric smoke detectors in addition to the noise light and the noise signals vary greatly by the change of an ambient temperature.
FIG. 9(A) shows the zero point levels of the photoelectric smoke detector when the ambient temperature is 0.degree. C., 25.degree. C. and 50.degree. C. As described above, the zero point level is at the signal level obtained by adding a photoelectric conversion signal (shown by the shaded portion) which is the output signal of noise light generated by the emission of the light emitting element to the noise signal of the light receiving elements and the amplifying circuit of received lights. Among these signals, the level of the noise signal is increased as the ambient temperature increases to 0.degree. C., 25.degree. C. and 50.degree. C., whereas, the level of the photoelectric conversion signal is unchanged.
Normal operation can be confirmed by setting a normal range between threshold values a, b on the both sides of the zero point level at the ambient temperature of 25.degree. C. In this case, the normal operation can be confirmed from that both the zero point levels at 0.degree. C. and 50.degree. C. which are obtained by the test emission of the light emitting element are within the normal range of the threshold values a, b.
On the contrary, when the light emitting element and the like operates abnormally, the abnormal operation can be determined from that the photoelectric conversion signal shown by the shaded portion becomes zero and the zero point level is below the threshold value a at any of 0.degree. C., 25.degree. C. and 50.degree. C.
However, the following problems arise when normal operation and abnormal operation are determined by the conventional test emission in the photoelectric smoke detector. First, S/N ratio is recently increased by sufficiently reducing the level of noise light resulting from light emission effected in the state of no smoke by the improvement of the smoke detecting structure of the photoelectric smoke detector. For example, the level of the photoelectric conversion signal as the output signal of noise light occupied in the zero point level is relatively reduced as compared with the level of an electric noise signal as shown in FIG. 9(B).
Consequently, when the normal range is set to a narrow area between the threshold values a', b' on the both sides of the zero point level at the ambient temperature of 25.degree. C. likewise the case of FIG. 9(A), since the zero point level at 0.degree. C. obtained by the test light emission of the light emitting element is below the threshold value a', the operation of the photoelectric smoke detector is erroneously determined abnormal, whereas erroneous determination is also made as to the zero point level at 50.degree. C. which exceeds the threshold value b', thus it is difficult to make correct determination.
On the contrary, when the normal range is increased as shown in FIG. 9(C), even if the light emitting element and the like are made abnormal and the photoelectric conversion signal shown by the shaded portions are made zero, the zero point levels at the ambient temperatures 25.degree. C., 50.degree. C. are not below the threshold value a" and within the normal range, even if the light emitting element and the like operate abnormally, they are erroneously determined to operate normally. As described above, it is difficult to determine normal operation and abnormal operation simply depending upon whether the zero point level is within a predetermined range or not.
Although it is contemplated to determine normal operation by setting threshold values at respective temperatures, its arrangement is made complex because the circumferential temperatures of the respective detecting elements must be measured.
As described above, when the level of the photoelectric conversion signal compared with the level of an electric noise signal. is lowered, there arises a problem as to whether the light emitting element, the light receiving element and the peripheral circuit are normal or abnormal can in fact be determined by the test operation carried out based on the zero point output whose normal range is set by the threshold values.